1. Field of the Invention
This invention relates to the treatment of an engine exhaust gas stream, and more specifically to a method and apparatus for treating an engine exhaust gas stream containing pollutants including unburned hydrocarbons.
2. The Related Art
Molecular sieves, including zeolites, have been found to be useful for adsorbing harmful components such as hydrocarbons from gaseous streams containing these and other pollutants. For example, U.S. Pat. No. 4,985,210 is directed to an exhaust gas purifying apparatus for an automobile employing a three-way catalyst with either a Y-type zeolite or a mordenite used in a hydrocarbon trap upstream of the three-way catalyst. In the embodiment of FIG. 2 of U.S. Pat. No. 4,985,210, a bed of activated carbon is disposed upstream of an adsorbent zone. A solenoid-operated valve mechanism serves to direct the exhaust gas stream either through or around the activated carbon bed, depending on the temperature of the exhaust gas stream, and then through the adsorbent zone and the three-way catalyst.
U.S. Pat. No. 5,051,244 is directed to a process for treating an engine exhaust gas stream in which the gas stream is directed through a molecular sieve in an adsorbent zone during the cold-start phase of engine operation. When the hydrocarbons begin to desorb, the adsorbent zone is by-passed until the catalyst is at its operating temperature, at which point the gas stream is again flowed through the adsorbent zone to desorb hydrocarbons and carry them to the catalyst zone. A paper by M. Heimrich, L. Smith and J. Kotowski entitled Cold-Start Hydrocarbon Collection for Advanced Exhaust Emission Control, SAE Publication Number 920847, discloses an apparatus which functions in a manner similar to that of U.S. Pat. No. 5,051,244.
U.S. Pat. No. 5,125,231 to Patil et al, dated Jun. 30, 1992, discloses an engine exhaust system for reducing hydrocarbon emissions, including the use of Beta zeolites as hydrocarbon adsorbents (see column 5, lines 63 through column 6, line 14). Zeolites having a silica/alumina ratio in the range of 70/1 to 200/1 are preferred adsorbents (see column 6, lines 7-11). The apparatus includes by-pass lines and valves to direct exhaust gases from a first converter directly to a second converter during cold-start operation and when the first converter reaches its lightoff temperature, to either by-pass the second converter or recycle effluent from it to the first converter (see e.g., column 10, lines 10-28).
U.S. Pat. No. 5,158,753 discloses an exhaust gas purifying device comprising: a catalyst device installed in the exhaust gas path of an internal combustion engine for treating the exhaust gas of the engine; an adsorbing device installed in the exhaust gas path between the catalyst device and the internal combustion engine, for treating the exhaust gas of the engine. One embodiment includes a heat exchanger for performing heat transfer between the exhaust gas flowing from the internal combustion engine to the adsorbing device and the exhaust gas flowing from the adsorbing device to the catalyst device. Alternatively, the catalyst device includes a catalyst secured in the low-temperature-side gas flow path of a heat exchanger, and the exhaust gas flowing from the internal combustion engine to the adsorbing device is allowed to flow to the high-temperature-side gas flow path of the heat exchanger.